class Node:
    def __init__(self, value):
        self.value = value
        self.lChild = None
        self.rChild = None
        self.parent = None
        self.bf = 0  # 左右子树高度差, 右 - 左
        

class VALTree:
    def __init__(self):
        self.root = None
        
    def rotate_left(self, ):
        """ 左旋 """